Aircraft need to produce varying levels of lift for take-off, landing and cruise. A combination of wing leading and trailing edge devices are used to control the wing coefficient of lift. The leading edge device is known as a slat. On larger aircraft there may be several slats spaced along the wing edge. During normal flight the slats are retracted against the leading edge of the wing. However, during take-off and landing they are deployed forwardly of the wing so as to vary the airflow across and under the wing surfaces. The slats usually follow an arcuate or curved path between their stowed and deployed positions. By varying the extent to which the slat is deployed along said path, the lift provided by the wing can be controlled.
An assembly is required to support and guide movement of a slat between stowed and deployed positions and a typical arrangement showing a cross-section through part of a wing 1 and a slat 2 in its stowed position is illustrated in FIG. 1. As can be seen from FIG. 1, the slat 2 is provided with an arcuate support arm or slat track 3, one end 4 of which is attached to the rear of the slat 2 and extends into the wing 1. To allow for wing bending and manufacturing tolerances, the end 4 of the slat track 3 is attached to the slat using spherical bearings and linkages (not shown). The slat track 3 penetrates wing spar 6 forming the wing structure. The slat track 3 defines an arc having an axis and is mounted within the wing so that it can rotate about that axis (in the direction indicated by arrows “A” and “B” in FIG. 1) to deploy and retract the slat 2 attached to one end of the slat track 3.
To drive the slat track 3 so as to deploy or retract the slat 2, a toothed slat track rack 7 having an arcuate shape corresponding to the arcuate shape of the slat track 3 is mounted within a recess 3a on the slat track 3 and a correspondingly toothed drive pinion 8 is in engagement with the teeth 7a on the slat track rack 7 so that when the drive pinion 8 rotates, the teeth 8a on the drive pinion 8 and the teeth 7a on the rack 7 cooperate to pivot or drive the slat rack 7 and the slat attached thereto, into a deployed position, i.e. in the direction of arrow “A” in FIG. 1. Typically, the slat track 3 rotates through an angle of 27 degrees between its fully stowed and fully deployed positions. Rotation of the pinion 8 in the opposite direction also drives the slat track 3, in the direction of arrow “B”, back into its stowed position, as shown in FIG. 1.
Each drive pinion 8 is mounted on an outer shaft of a geared rotary actuator which extends concentrically over an inner input drive shaft that extends along the length of the wing within its leading edge, and which is driven by a slat deployment motor coupled to the inner input drive shaft at an inboard end of the wing. The inner input drive shaft is a common input drive shaft so that the slat deployment motor is operable to deploy all the slats of one wing together. The geared rotary actuator couples the inner input shaft to the outer output shaft so that the output shaft is driven by the inner input shaft. The rotary actuator also controls the speed of rotation of the output shaft relative to the input shaft so that the output shaft rotates approximately 200 to 300 times slower than the input shaft. A separate rotary actuator is associated with each drive pinion 8 and its accompanying slat track rack so there may be two or more geared rotary actuators per slat spaced along the length of the wing and extending concentrically over the input shaft.
It is important to ensure that all the slats are deployed together at the same rate, within a defined tolerance, so as to prevent any skewing or asymmetry incurred as a result of inconsistent slat deployment. Skewing of a slat occurs when one of a number of slat deployment mechanisms associated with the same slat fails so that the slat deploys at an angle because it is still being driven away from the leading edge of the wing at an angle by the remaining slat deployment mechanism(s) associated with that slat. Asymmetry occurs when the slats on one wing are deployed at a different rate or extent to the slats on the other wing. Asymmetry or skewing of slats can be caused as a result of, for example, a defective rotary actuator, common drive shaft or coupling between the inner shaft and an outer shaft of a rotary actuator.
The present invention seeks to provide a system for determining whether the relative rate of deployment of all the slats extending from a leading edge of an aircraft wing is the same as a predetermined relative rate of deployment so that further slat deployment can be prevented when the predetermined relative rate differs from the detected relative rate. If the detected relative rate of deployment differs from a predetermined rate, it can be assumed that skewing or asymmetric slat deployment is occurring and steps can then be taken to prevent further deployment of the slats.